The document discusses the mechanisms of urine concentration and dilution. It explains that 180L of glomerular filtrate is formed daily but urine needs to be concentrated to not deplete the body's water stores. Urine concentration depends on water content and antidiuretic hormone (ADH). Dilute urine forms when water increases by inhibiting ADH, preventing water reabsorption. Concentrated urine forms when water decreases by developing a medullary osmotic gradient maintained by countercurrent exchange in the loop of Henle and vasa recta, and secreting ADH to allow water reabsorption in the collecting duct.

2. INTRODUCTION:
• Every day 180 L of glomerular filtrate is formed with large quantity of water.
• If this much of water is excreted in urine, body will face serious threats.
• So the concentration of urine is very essential.
• Osmolarity of glomerular filtrate is same as that of plasma and it is 300
mOsm/L.
• But, normally urine is concentrated and its osmolarity is four times more
than that of plasma, i.e. 1,200 mOsm/L.
• Osmolarity of urine depends upon two factors:
1. Water content in the body.
2. Antidiuretic hormone (ADH).
• Mechanism of urine formation is the same for dilute urine and concentrated
urine till the fluid reaches the distal convoluted tubule.

3. FORMATION OF DILUTE URINE:
A. When, water content in the body increases,
kidney excretes dilute urine.
B. This is achieved by inhibition of ADH secretion
from posterior pituitary.
C. So water reabsorption from renal tubules does
not take place leading to excretion of large
amount of water.
D. This makes the urine dilute.

4. Mechanism action of Formation
of Dilute Urine:
1. Tubular Fluid Remains Isosmotic (300 mOsm/L) in the
Proximal Tubule:
• Solutes and water are reabsorbed in equal proportions.
2. Tubular Fluid Is Diluted in the Ascending Loop of Henle
(100 mOsm/L):
• Sodium, potassium, and chloride are avidly reabsorbed
also this portion of the tubular segment is impermeable to
water.
3. Tubular Fluid in Distal and Collecting Tubules Is Further
Diluted (50 mOsm/L) in the Absence of ADH:
• Additional reabsorption of sodium chloride and In the
absence of ADH, this portion of the tubule is also
impermeable to water,

6. FORMATION OF CONCENTRATED
URINE:
• When the water content in body decreases,
kidney retains water and excretes
concentrated urine.
• Formation of concentrated urine is not as
simple as that of dilute urine.
• It involves two processes:
1. Development and maintenance of medullary
gradient by countercurrent system.
2. Secretion of ADH.

7. MEDULLARY GRADIENT:
• MEDULLARY HYPEROSMOLARITY:
• Cortical interstitial fluid is isotonic to plasma with the
osmolarity of 300 mOsm/L.
• Osmolarity of medullary interstitial fluid near the
cortex is also 300 mOsm/L.
• However, while proceeding from outer part towards
the inner part of medulla, the osmolarity increases
gradually and reaches the maximum at the inner
most part of medulla near renal sinus (hypertonic
1,200 mOsm/L).
• It plays an important role in the concentration of
urine.

9. • DEVELOPMENT AND MAINTENANCE OF
MEDULLARY GRADIENT:
• Kidney has some unique mechanism called
countercurrent mechanism, which is
responsible for the development and
maintenance of medullary gradient and
hyperosmolarity of interstitial fluid in the
inner medulla.

10. COUNTERCURRENT
MECHANISM:
• A countercurrent system is a system of
‘U’shaped tubules (tubes) in which, the flow
of fluid is in opposite direction in two limbs
of the ‘U’shaped tubules.
• Divisions of Countercurrent System:
1. Countercurrent multiplier formed by loop of
Henle.
2. Countercurrent exchanger formed by vasa
recta.

11. 1- COUNTERCURRENT MULTIPLIER
• Loop of Henle:
i. The active reabsorption of sodium chloride and other
solutes from ascending limb of Henle loop into the
medullary interstitium.
• Then sodium chlorine ions diffuse from medullary
interstitium into the descending limb of Henle loop.
• Thus sodium chlorine ions are repeatedly recirculated
between the descending limb and ascending limb of Henle
loop.
• Other Factors Responsible for Hyperosmolarity of
Medullary Interstitial Fluid:
ii. Reabsorption of sodium from collecting duct.
iii. Recirculation of urea.

12. • 50% of urea reabsorbed in proximal convoluted
tubule.
• So the fluid in distal convoluted tubule has as much
urea as amount filtered.
• Collecting duct is
impermeable to urea.
• However, due to the
water reabsorption
from distal convoluted
tubule and collecting
duct in the presence of
ADH, urea
concentration increases
in collecting duct.

13. 2- COUNTERCURRENT
EXCHANGER:
• Vasa Recta:
• It is also ‘U’shaped tubule with a descending limb,
hairpin bend and an ascending limb.
• Vasa recta runs parallel to loop of Henle.
• Its descending limb runs along the ascending limb
of Henle loop and its ascending limb runs along
with descending limb of Henle loop.
• Vasa recta functions as countercurrent exchanger.
• It is responsible for the maintenance of medullary
gradient, which is developed by countercurrent
multiplier.

14. MOA of VASA RECTA
1. The sodium chloride reabsorbed from ascending limb of
Henle loop enters the medullary interstitium.
2. From here it enters the descending limb of vasa recta.
3. Simultaneously water diffuses from descending limb of
vasa recta into medullary interstitium.
4. The blood flows very slowly through vasa recta.
5. Recycling of urea also occurs through vasa recta.
6. From medullary interstitium, along with sodium chloride,
urea also enters the descending limb of vasa recta.
• Sodium chloride and urea are exchanged for water
between the ascending and descending limbs of vasa
recta, hence this system is called countercurrent
exchanger.

17. Mechanism Action Of
CONCENTRATED URINE:
1. BOWMAN CAPSULE:
• Osmolarity of the filtrate at Bowman capsule is {300
mOsm/L}.
2. PROXIMAL CONVOLUTED TUBULE:
• Reabsorption of sodium and chloride followed by
obligatory reabsorption of water {isotonic to plasma
(300mOsm/l)}.
3. THICK DESCENDING SEGMENT:
• Water is reabsorbed from tubule into outer medullary
interstitium by means of osmosis {hypertonic to
plasma (450 and 600 mOsm/L)}.

18. 4. THIN DESCENDING SEGMENT OF HENLE LOOP:
• More water is reabsorbed {hypertonic to plasma (1,200
mOsm/L)}.
5. THIN ASCENDING SEGMENT OF HENLE LOOP:
• Osmolarity decreases gradually due to sodium chloride diffuses
out of tubular fluid {hypertonic to plasma (400 mOsm/L)}.
6. THICK ASCENDING SEGMENT:
• This segment is impermeable to water & reabsorption of sodium
chloride {hypotonic to plasma (150 and 200 mOsm/L)}.
7. DISTAL CONVOLUTED TUBULE AND COLLECTING DUCT:
• In the presence of ADH, permeable to water resulting in water
reabsorption and final concentration of urine {hypertonic to
plasma (1,200 mOsm/L)}.

21. Acidification of Urine

22. • Normally, urine is acidic in nature with a pH of
4.5 to 6.
• Metabolic activities in the body produce large
quantity of acids (with lot of hydrogen ions),
which threaten to push the body towards
acidosis.
• However, kidneys prevent this by two ways:
1. Reabsorption of bicarbonate ions (HCO3–)
2. Secretion of hydrogen ions (H+).

23. 1- REABSORPTION OF
BICARBONATE IONS:
• About 4,320 mEq of HCO3 – is filtered by the
glomeruli everyday.
• It is called filtered load of HCO3 –.
• Excretion of this much HCO3 – in urine will
affect the acid-base balance of body fluids.
• So, HCO3 – must be taken back from the renal
tubule by reabsorption.

24. SECRETION OF HYDROGEN
IONS:
• About 4,380 mEq of H+ appear every day in the renal tubule by
means of filtration and secretion.
• 50 to 100 mEq is excreted, It results in the acidification of
urine.
• Secretion of H+ into the renal tubules occurs by the formation
of carbonic acid.
• Secretion of H+ occurs by two pumps:
i. Sodium-hydrogen antiport pump
ii. ATP-driven proton pump.
• Excretion of H+ occurs by three mechanisms:
1. Bicarbonate mechanism
2. Phosphate mechanism
3. Ammonia mechanism.

26. Metabolic acidosis occurs when kidneys fail to excrete metabolic acids.
Metabolic alkalosis occurs when kidneys excrete large quantity of hydrogen.